General speaking, the dissolution and recovery of platinum group metals (PGM's), was thought to be possible only by the use of aggressive chemicals. A well known process is dyanide dissolution or platinum group metals.
Dissolution of platinum group metals with a cyanide solution, e.g., sodium cyanide solution, is relative easy, particularly the dissolution of PGM's in their elemental state to form cyano-complexes that are stable in aqueous solutions. However, if PGM's exist in mineral form, such as in oxide ores, it may be necessary to liberate the precious-metals, for example, by converting them into their elemental states or other soluble form before dissolving them with a cyanide solution.
A disadvantage of cyanide dissolution is that it presents environmental problems and particular care must be taken in disposing of waste solutions. Moreover, cyanides are costly materials which make their use economically undesirable.
Another aggressive chemical employed to dissolve precious metals is aqua regia. Aqua regia insures dissolution of PGM's, because of its high acidity, its high redox potential (due to the presence of nitric acid), and its high concentration of complexing ions (12M chloride ions) for complexing the precious metals.
However, the use of aqua regia has its economic disadvantages because of its unacceptable high reagent use, cost and consumption.
A process has been reported regarding the use of bromine for leaching gold from ores. It was found that a relatively low bromide ion concentration can be used to effect dissolution of elemental gold, for example, 0.1M bromide ions, as compared to the much higher chloride ion concentration (12M chloride ions) required for dissolution with aqua regia.
In this connection, reference is made to an article by Batric Pesic et. al. entitled Dissolution of Gold with Geobrom 3400, Fundamental and Applied Studies, which was delivered at the Proceedings of the 13th IMPI Conference at Montreal Precious Metals 1989.
Geobrom 3400 (which is the trademark of a product of Great Lake Chemical Corporation) is a solution containing about 34% free bromine. According to the aforementioned article, the bromine solution was used to dissolve elemental gold from gold concentrates and electronic scrap. Rotating disc studies were conducted using a rotator, a speed controller, a reactor, and a water bath. A typical experiment comprised a 500 ml solution containing 5 ml/l of Geobrom 3400 at a natural pH at a temperature of 250.degree. C., the stirring being carried out at 500 rpm.
The studies indicated that bromine provided a substantially higher rate of gold dissolution than dissolution with sodium cyanide or thiourea.
The use of bromine for the dissolution of gold is disclosed in another article entitled Leaching and Recovery of Gold From Black Sand Concentrate and Electrochemical Regeneration of Bromine by A. Dadgar et. al. This article was presented before the Society for Mining, Metallurgy, and Exploration, Inc. at Reno, Nev. on Sep. 10-12, 1990. This paper describes the leaching of gold with Geobrom 3400 from very rich black sand concentrate and its subsequent recovery by ion exchange resins and solvent extraction.
Mineralogical examinations of PGM in ores generally reveal native metals, sulphides, tellurides, arsenides and antimonides, etc., associated with base metal sulphides as occlusions, intergrowths and solid solution in gabbroic rocks. In South Africa over 20 PGM minerals have been identified, the most common of which are sperrylite (PtAs.sub.2), cooperite (PtS), braggite ((Pt, Pd, Ni) S), and a Pt--Fe alloy. Braggite, vysotskite (PdS) and Pt-Fe alloys are common in an ore known as the Stillwater ore, while Sudburyite (PdSb) is found in Kambalda and Sudbury ores. In the more oxidized Coronation Hill and alluvial deposits the majority of the PGM is present as native Pt and Pd, sometimes alloyed with Fe, associated with the heavier minerals such as magnetite and milenite.
A particular ore of interest is an oxide ore referred to as The Hartley Complex of the Great Dyke in Zimbabwe. Estimates of the value of all Great Dyke ore bodies indicate that it contains approximately 800 million ounces of platinum and 19 million ounces of rhodium. However, since 9% of the ore is oxidized and cannot be processed with current recovery methods, it is discarded as waste which represents a loss of many millions of dollars.
The ore contains at least one precious metal, especially platinum, selected from the group consisting platinum group metals, gold and silver, some in the elemental form, but usually in the form of a precious metal compound selected from the group consisting of sulfides, arsenides, tellurides, selenides, antimonides, and bismuthinides, or as a ferro-alloy of the precious metal, e.g., Pt--Fe alloy. Other ores include certain gold ores containing carbon, generally referred to as refractory ores.
Because the PGM content of the Hartley ore is quite low, mineralogical investigations were carried out on samples concentrated either by shaking table or heavy liquid separation. Mineralogically, the ore contains PGM'S, silicate gangue minerals, oxides and base metal (BM) sulfides. The ore minerals, making up less than 0.01 vol. % of the ore, constitute silicate gangue minerals with the remainder of the ore made up of oxides and BM sulfides.
The gangue minerals consists primarily of hydrated silicates, talc being the main component. Substantial amounts of tremolite, serpentine and chlorite are present. The mineral oxides comprise iron (II) oxide, ilmenite, chromite and wolframite, while the sulfides comprised pyrrhotite (FeS) , chalcopyrite (CuFeS.sub.2) and pentlandite (FeNiS).
We have discovered a method for the recovery of PGM's from ores of the aforementioned types by simply using a sulfuric acid solution relatively low in halide concentration and of controlled redox potential at least sufficient to convert the precious metal to an ionic form conducive to forming a soluble bromide complex thereof.